Vacuum coating method

ABSTRACT

IN VACUUM METALLIZING, CREEP WITHIN A CRUDIBLE IS INHIBITED BY HEATING THE BODY PORTION OF THE CRUCIBLE TO A TEMPERATURE ABOVE THE VAPORIZATION TEMPERATURE OF THE METAL COATING STUFF WHILE HEATING A VERTICALLY EXTENDING LIP PORTION OF THE CRUCIBLE TO A TEMPERATURE WH IS BELOW SAID VAPORIZATION TEMPERATURE BUT ABOVE THE MELTING POINT TEMPERATURE OF SAID METAL COATING STUFF.

W3 J. T. CARLETON 3,736,175

VACUUM COATING METHOD Original Filed Dec. 16, 1970 EIGQI INVENTOR JOHN T. CARLETON ATTORNEY United States Patent 3,736,175 VACUUM COATING METHOD John T. Carleton, Upper Arlington, Ohio, assignor to I13). I. du Pont de Nemours and Company, Wilmington,

el. Continuation of abandoned application Ser. No. 98,824, Dec. 16, 1970. This application June 2, 1972, Ser.

Int. Cl. C23c 17/02 US. Cl. 117-107.1 1 Claim ABSTRACT OF THE DISCLOSURE In vacuum metallizing, creep within a crucible is inhibited by heating the body portion of the crucible to a temperature above the vaporization temperature of the metal coating stuff while heating a vertically extending lip portion of the crucible to a temperature which is below said vaporization temperature but above the melting point temperature of said metal coating stuff.

This is a continuation, of application Ser. No. 98,824, filed Dec. 16, 1970 and now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to vacuum metallizing apparatus and, more particularly, to the melting and vaporization of materials in a crucible in a vacuum environment and by induction heating.

Description of the prior art The use of induction heating for vaporizing metal contained in a crucible in the vacuum metallizing apparatus arts is old.

The material to be vaporized is placed in a crucible which, in turn, is placed within a heating coil of an induction heating means located in the vacuum chamber. The chamber is pumped to a suitable vacuum, and normally a high frequency alternating current is applied to the coil by which the crucible and metal charge are heated inductively to vaporizing temperatures. With a number of materials the heated molten metal or melt tends to creep up the sides of the crucible, onto the lip portion, and in some instances may wick over the edges into the insulation between the heating coils and the crucible. With reactive materials, such as aluminum, the coils may be attacked and the useful life shortened. Further, the crucible and insulating attack may lead to undesirable contaminants in the vacuum chamber.

One approach to solving this problem is shown in US. Patent 2,665,320 to Chadsey, I r. et al., in which the crucible lip is heated to a substantially highter temperature than the molten metal in the crucible so that the lip tends to vaporize the molten metal reaching the lips. This approach has certain disadvantages; for example, the lip radiates added heat in the chamber which must be removed by additional cooling of the structure of the chamber and the substrate or object to be coated and, more importantly, the material vaporized from the lip may travel in all directions above the lip where it may deposit in undesired areas rather than on the object to be coated. This is not only wasteful of material but increases the downtime for cleaning the chamber.

SUMMARY OF THE INVENTION The vacuum metallizing apparatus illustrating the method of the present invention provides a means for heating a crucible in a novel manner thereby to improve the efficiency of the apparatus.

Briefly described, a crucible cointaining metal to be vaporized is positioned within a vacuum chamber of the vacuum metallizing apparatus and the lip or lip portion of such crucible is heated so it does not vaporize the metal which is vaporized by the heat supplied to the main body portion of the crucible.

This invention solves a problem existent in the prior art by providing an improved method for controlling melt (molten metal) creeping during melting and vaporization.

Such invention essentially encompasses the discovery of the improvement of heating the main body portion of the crucible hotter than the lip portion to control creeping of the molten material up the wall of the crucible toward the lip portion thereof.

It is known that certain metals when vaporized have a tendency to wet and creep up the sides of the crucible which cointains them and in which they are melted (aluminum vaporized in a graphite crucible, for example). Such a tendency is more pronounced when the crucible is heated by induction coils and the rim of the crucible is insulated to protect the coil and to increase thermal efficiency.

One of the prime disadvantages of creeping is that the molten metal on the sides of the crucible tends to be deposited onto the object to be metallized in a direction other than the desired vertical direction, a step of crucial and major significance in the deposition of any objection.

By locating the crucible lip portion way from the coil, the heat generated in the lip is substantially reduced and, by further removing the insulation from the lip, the heat differential between the lip portion and main body portion of the crucible is further enhanced. This heat differential tends to decrease the tendency of the liquid (molten) metal being vaporized to creep along and over the edges of the crucible and, thus, be vaporized other than in the proper direction, thus bringing about a substantial gain in increased efficiency of the deposition step.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic sectional view of the vacuum metallizing apparatus of this invention, with parts omitted for clarity, showing a web of sheet material being coated in a vacuum chamber by vaporized metal melted and vaporized in a crucible by means of an induction heating means surrounding the main body portion of the crucible but positioned below the lip portion thereof.

FIG. 2 is a sectional view of the crucible, the insulation therefor and the induction heating means for heating such crucible in a novel manner and in the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing and particularly FIG. 1, a web it) of sheet material (such as plastic film) to be coated by the web coating or vacuum metallizing apparatus of this invention is supplied from a supply source, such as supply roll 11, and moved into operative association with means 12 for coating the web 10 in a manner to be described. From the supply source 11, the web or substrate 10 passes over transfer rolls 13 and 14 and onto a windup roll 15, all of which parts may be appropriately mounted in a vacuum chamber of the metallizing apparatus of this invention, as is known.

In the drawing, there are shown various parts of a typical vacuum metallizing apparatus .20 which includes a gas-impervious body member or wall 21 defining a vacuum chamber 22. Within the vacuum chamber 22 and mounted on appropriate support means 23 is a container or crucible 24 in which is contained material 25' (as shown in FIG. 2) to be vaporized so as to produce metal vapor 25' for coating the moving web 10.

The container or crucible 24, wherein the material or metal 25 to be vaporized is located, has a mouth or opening positioned between the transfer rolls 13 and 14 which positions the moving web adjacent the crucible opening during the metallizing operation. The crucible 24 supports therein a pool of the material 25, such as molten aluminum, in the appropriate position to supply the stream of aluminum vapors 25 upwardly to condense on the moving web 10 upon the melting and vaporization of the metal 25.

Means are provided to melt and vaporize the metal 25 in the crucible 24 in the form of an induction heating means 30 mounted in the body member 21 of the metallizing apparatus. The metal 25, upon vaporization, deposits and condenses on the web 10 near the outlet of the crucible 24 where it condenses to form a coating.

The vacuum chamber 22 is coupled through a port 46 to a source of high vacuum 26 for evacuating the vacuum chamber 22 as is known in the art.

Crucible 24 is of an electrically conductive material, such as graphite, and it rests on the base or support 23 which is insulated.

Insulating base 23 is preferably of a heat-resistant ceramic material so that occasional minor spillover of the metal in startup will not cause damage.

The chamber 22 has means defining a viewing portion including a glass 27 positioned over a port 46 and a rotatable shield 28 operable from outside the chamber 22 which is used to protect the glass 27 from being coated when not used for operation.

A power supply 31 provides the energy for induction heated means inside the vacuum chamber 22. The frequency of the power supply may vary from 1,000 to 500,000 hertz or more depending upon the arrangement and materials of construction.

The crucible 24 containing the material 25 to be vaporized is surrounded by insulation 33, 34 which is preferably essentially not inductively heated by the induction field provided 'by the heating means 30.

The web substrate 10 may be a film together with roll mechanisms to unwind and wind up the film. There may further be cooling plates or rolls behind the substrate or film. Other well-known items such as masks to control deposition on the substrate, baflies, shutters, and cooling coils may be present. These are not shown as they are not basic to the invention.

FIG. 2 is a detailed sectional view of the crucible, insulation and heating means. The crucible 24 is constructed of suitable material such as carbon, or metal carbides and nitrides which may be heated by eddy currents. A suitable material is graphite or a combination of graphite and carbon. The crucible 24 may be made from a hollowed-out cylinder, half cylinder, open box, or other suitable shape. The walls may be coated to prolong the life and prevent attack of the molten charge 25 upon the crucible.

The basic inventive concept is applicable to a number of materials, but it is particularly directed to metals and molten aluminum.

The insulation and support materials 33 and 34 separate the crucible 24 from coils 35 of the heating means 30. A satisfactory combination is a fibrous or powdery material which may be graphite fibers, aluminum oxide, or zirconium oxide. The outer layer may be built of ceramic paper such as Fiberfrax (The Corborundum Company). The induction coils 35 are made of a conductive material such as copper and may be covered with a tape such as fiberglass and cemented in place around the outer insulation 34, as best seen in FIG. 1. The coils 35 are preferably hollow to permit the circulation of coolant.

The crucible consists of a main body portion 36 and a lip portion 37.

The lip portion 37 of the crucible 24 is raised above the upper edges 33' and 34' of the insulation 33 and 34 and above the top of the uppermost induction heating coil element 35'. Raising the lip 37, thus causes it to be and remain cooler.

At temperatures of 1,000 C. or more, sufiicient cooling may occur if the crucible lip 37 is raised less than one inch. It is to be remembered that the loss of heat by radiation varies with the fourth power of the absolute temperature, and the edge may be 200 C. or more cooler than the melt. With lower melting materials, it may be desirable to have a heat sink nearby or contacting the edge to provide the necessary temperature gradient without having the crucible edge unduly raised. It is within the concept of this invention to provide such additional cooling over the natural emission or to vary the crucible height and shape to accentuate the temperature difference.

The desired temperature difference between the molten material 25 (melt) and the lip or lip portion 37 can be determined by simple experimentation to suit the material and particular configuration of crucible and heater. The lip 37 should be above the melting point of the material 25 being vaporized, otherwise, condensation of the vapor will occur and plugging or choking of the crucible mouth may occur. Once the crucible height or heat sink condition has been established, further adjustment is not required. A few trial runs where the lip 37 is extended different distances are usually sufficient to establish the conditions. If the melt creeps to the lip 37 of crucible 24, vaporization spreads over the chamber 22 coating the walls and the window glass 27 if the cover 28 is opened this glass 27 may be used (for observation of the crucible edge with optical or infrared pyrometers.

Induction heating is heating material by means of an electric current that is caused to flow through the material or its container by electromagnetic induction. It is known that those crucibles made of materials such as graphite and combinations of graphite and carbon, while relatively inert to molten aluminum, also are readily wetted by such molten aluminum. In a crucible of this type, it has been found that the aluminum has a tendency, due to its ability to web the material of crucible, to climb up the side of the crucible and over the edge or lip thereof. This results in wasting the material, a reduction of metallizing efliciency and other problems. The solving or minimizing of this creeping problem is the touchstone of this inventlon.

The heating means in essence comprises an induction coil which is capable of inducing eddy currents in the metal 25 and in crucible 24 to heat the metal to the desired vaporizing temperature. Extending from the main body portion 36 of the crucible 24 is the lip portion 37. Essentially, the induction coil stops at the upper edge of the main body portion 36, leaving the lip portion 37 freely extending thereabove.

Since the lip portion 37 is at a lower (nonvaporizing) temperature, the material in contact with such lip portion will not be vaporized thereby and will be prevented from creeping over the upper edge or lip of the crucible.

The lower temperature of the lip portion 37, with its direct effect on the viscosity of the molten (liquid) material 25, slows the movement of such liquid material 25 proportionately and, thus, decreases or stops creeping of such liquid up the walls of the crucible 24. Further, the material 25 that does creep up the walls does not vaporize due to the non-vaporizing temperature of such walls and, hence, the vapors originate only from the proper areas of the crucible 24.

In the operation of the apparatus schematically illustrated, the web 10 which may be paper, plastic or the like, is positioned between the transfer rolls 13 and 14 in the coating chamber. The web 10 can be introduced from the outside through appropriate seals (not illustrated) or it can be mounted in the vacuum chamber 22. The vacuum chamber 22, where the vapor coating of the Web 10 occurs is preferably pumped down by pump to a pressure of less than 1 micron Hg Abs.

The surface of the metal 25, such as aluminum, is raised by the heating means 30 to a very high temperature-on the order of 1200 to 1300 C., so copious quantities of aluminum vapors 25' are released. Due to the high vacuum in the coating chamber 22, the aluminum vapors 25' travel in substantially straight lines from the surface of the aluminum pool 25. Most of these vapors 25 condense on the web 10 which is moved adjacent the top opening of the crucible 24 containing the metal 25.

The following examples will aid to further point out the invention.

EXAMPLE 1 A graphite crucible 15.5 inches long by 5 inches wide with a wall thickness of inch was arranged with the top edge of crucible at the same level as the insulation and top coil of the induction heater in a vacuum chamber. The crucible surface had previously been coated with boron nitride to prevent attack of aluminum on the graphite surface. The crucible was charged with aluminum to within one-half inch of the rim. The vacuum chamber was pumped down to torr. The induction heater was operated at 9600 hertz. The crucible and contents Were raised to evaporating temperature (about 1400" C.). The aluminum crept up the sides of the crucible and evaporated from the edge. When the window was used to observe the evaporation, it readily fogged with aluminum deposit indicating wide-spread distribution of the aluminum vapor.

EXAMPLE 2 The procedure was repeated as for the comparative example except the crucible edge was raised one-half inch above the insulation and coil top. When evaporation was started, the process was observed through the window. The window fogging required about 10 times as long as in the comparative example. The aluminum did not creep over the edge. The edge was estimated to be about 200 C. cooler than the melt. By raising the crucible edge, the power input was slightly increased but this was more than compensated for by the decrease in chamber contamination and life of the induction coil.

By the practice of this invention, the coating efiiciency of a vacuum metallizing apparatus may be dramatically improved due to the solving or minimizing of the problem of creeping of the melt during the vaporizing operation.

I claim:

. 1. A method of vacuum metallizing wherein an object to be metallized is placed within a vacuum chamber adjacent a crucible containing the metal to be vaporized including the step of heating, by induction heating, the main body portion of the crucible to a temperature above the vaporizing temperature of the metal thereby to form vapors to metallize the exposed surface of the object as the object is advanced past the crucible while heating a vertically extending lip portion of the crucible to a temperature lower than the vaporizing temperature of the metal and above the melting temperature of such metal.

References Cited UNITED STATES PATENTS 2,665,224 1/1954 Clough et al 118-49 X 3,029,777 4/1962 Cerych et a1 11849 3,034,924 5/1962 Kraus et al 118-49 X 3,330,252 7/1967 Oberg l1849.l

3,572,672 3/1971 Havel 11848 X FOREIGN PATENTS 685,267 10/1952 Great Britain 118-48 OTHER REFERENCES IBM Technical Disclosure Bulletin, RF Heated Crucible for Aluminum Evaporation, Ames & Kaplan, vol. 6, No. 8 (January 1964), p. 103.

IBM Technical Disclosure Bulletin, Shaped Crucible," Roland, VOL. 9, N0. 6 (November 1966), p. 692.

MORR IS KAPLAN, Primary Examiner U.S. Cl. X.R. 118-49.5 

